Ni/MH batteries in which a hydrogen-absorbing alloy is used as a negative electrode material were commercialized in the first half of the 1990's, and their use subsequently spread widely.
Ni/MH batteries were initially employed as electric power supplies of mobile phones and personal computers.
Since then, small and lightweight lithium ion batteries have gradually replaced Ni/MH batteries in these fields.
Ni/MH batteries are now used in toys, digital cameras, motor-assisted bicycles, power tools, and hybrid electric vehicles because they have a good balance among their characteristics such as economy, safety, and electric energy density.
Among such batteries, large and prismatic Ni/MH batteries for electric drive motors of hybrid electric vehicles particularly need a high level of performance.
First of all, the Ni/MH batteries for electric drive motors of hybrid electric vehicles need excellent high-rate discharge characteristics in order to drive electric motors which move a vehicle body weighing more than 1 ton.
Additionally, since the conditions in which hybrid electric vehicles are used are much more severe than the conditions in which an ordinary household appliance is used, it is essential that the batteries maintain high power at low temperatures, such as in a subfreezing environment.
Furthermore, because batteries for hybrid electric vehicles are functional components affecting human lives, the batteries must guarantee a much higher degree of safety for a prolonged period than batteries used in portable devices.
In order to obtain Ni/MH batteries having such excellent characteristics, the hydrogen-absorbing alloy used as a negative electrode material of the batteries must have a specially designed composition.
A general-purpose hydrogen-absorbing alloy used for Ni/MH batteries is an R-based R—Ni—Co—Al—Mn alloy system, in which R is a mixture of rare earth metals.
This alloy is an AB5-type hydrogen-absorbing alloy having a CaCu5-type crystal structure, and its typical composition is LaNi5.
This alloy is a multi-component alloy in which the balance among various characteristics can be adjusted by the substitution of a part of La by light rare earth elements such as Ce, Pr, and Nd, and/or by the substitution of a part of Ni by metallic elements such as Co, Al, and Mn.
However, it is generally considered to be difficult for the basic R—Ni—Co—Al—Mn alloy (referred to below as the “basic alloy composition”) to improve battery characteristics as a whole only by adjustment of the molar ratio of each element.
For example, the easiest way to obtain Ni/MH batteries having a high power density when an alloy having the basic alloy composition is employed is to decrease the content of Co and/or to increase the content of Mn.
However, it is well-known that doing so worsens other characteristics of Ni/MH batteries, especially the cycle life.
Therefore, when an alloy having the basic alloy composition is employed as a negative electrode material of Ni/MH batteries, it is typical to select certain characteristics as critical taking into consideration the application of the batteries and then to employ an alloy composition which can improve only the selected characteristics.
Thus, even if a certain alloy is categorized as “an alloy having the basic alloy composition”, the molar ratio of each element of the alloy can vary widely depending upon which characteristics of the batteries are selected as critical, and hence the characteristics of the alloy are naturally different from each other.
Also, it is often the case that an alloy applied to a certain use has excellent characteristics which are selected for that use while the other characteristics are allowed to be inferior.
For example, an alloy which exhibits excellent cycle life for some applications may not have a good energy density and/or high-rate characteristics.
Conversely, some applications may require an alloy to have good high-rate discharge characteristics even if the cycle life of the alloy is not so good.
However, there are many cases in which several characteristics of an alloy must be simultaneously improved in order to meet the requirements of the application of the alloy.
To respond to the need to simultaneously improve several characteristics, it has been proposed to add one or more elements other than elements constituting the basic alloy composition, or to adjust some control parameters of characteristics other than the alloy composition even when the alloy has the basic alloy composition of the basic alloy composition.
For example, Patent Document 1 discloses selecting a non-stoichiometric composition in which the B site is relatively rich, and at the same time, adding a small amount of elements such as Fe, Cr, and Cu.
Patent Document 2 suggests reducing impurities such as Mg, Pb, and Cl in order to improve the high-rate discharge characteristics, i.e., the high-rate characteristics of the alloy.
Patent Document 3 discloses an alloy, which has the basic composition, has a crystal structure which is defined precisely so as to improve several characteristics.    Patent Document 1: JP09-213319A    Patent Document 2: JP10-25529A    Patent Document 3: WO2003/054240